Fractionation tray with adjustable capacity weir means

ABSTRACT

In accordance with the present invention, overflash within a withdrawal zone of a complex fractionating column is monitored to provide a surprisingly useful control characteristic related to the efficiency (or lack thereof) of not only the fractionation column inself, but associated processes linked to the column. To accurately monitor the overflash, notched weir means including an upright wall means, is positioned at the intersection of the withdrawal tray and the downcomers connecting the former with a lower, adjacent fractionating tray. Differential pressure measuring means is also provided with sensing means in the vicinity of the notched weir means. In turn, the differential pressure measuring means is connected in series to a recording means located external of the column. Result: As overlimit swings in overflash occur, corrective action, say via a separate process controller means, can be quickly implemented to bring the system back to stable operations.

This application is a division of application Ser. No. 449,150, filedMar. 7, 1974 now U.S. Pat. No. 3,985,623.

FIELD OF THE INVENTION

The present invention relates to an apparatus and method for monitoringliquid condensate flow within a complex fractionation processing system,and more particularly to a method and means which utilizes the flow rateof excess overflash within a complex fractionation process as a controlcharacteristic of not only the fractionation column itself but also ofassociated processes linked to the column. In this application, theterms "complex distillation column" and "complex fractionation column"refer to a column that has two or more feeds and/or one or morewithdrawal streams in addition to conventional distillate and bottomproducts. See Encyclopedia of Chemical Technology, 2nd Ed., Vol. 7, p.231.

BACKGROUND OF THE INVENTION

Today's petroleum refinery processes include many complex operations,one or more of which may require controlled regulation of processstreams as a function of measured system parameters. For example, in acomplex hydrocarbon flash distillation process, an intermediate liquidside stream drawn from liquid condensate within a complex distillationcolumn at, say a withdraw zone, may be used for multiplicity ofpurposes, inter alia, (i) to produce subsequent jet fuel product in adownstream liquidvapor stripping process, (ii) to control flow, andhence, efficiency in side stream gas recovery columns, reboilers, andthe like, and (iii) to control intermediate recycle circulation of thedistillation column itself. Remaining excess liquid condensate withinthe column, called overflash, is free to cascade down to lowerfractionation trays of the column located below the withdrawal zone. Inproviding items (i)-(iii), above, the flow rate of the side streamexiting from the column has been found to be a valuable parameter forcontrol purposes. However, experience also has shown that if theestablished withdraw rate becomes unbalanced vis-a-vis the flow rate ofthe overflash at the withdrawal zone, there can be upsets in theassociated processes linked to the withdrawal line as well as areduction in fractionation efficiency within the distillation columnitself. See Shinskey, F. G., "The Value of Process Control" Oil and GasJournal, Feb. 18, 1974 and articles cited therein.

While instrumentation for measuring process parameters in the externalpiping is feasible, response time for such equipment is often inadequateto properly control the complex processing system, at least in thesituation where deviation from selected setpoint limits occurs at arather rapid rate.

OBJECT OF THE INVENTION

An object of the invention is the provision of a novel system forcontrolling a complex fractionation system by which a surprisinglyuseful control characteristic associated with internal overflash flow inthe withdrawal zone of a complex distillation column can be easilygenerated. Once deviation from acceptable setpoint limits is detected,the overflash control operator aids in reestablishing acceptableprocessing control parameters within the system.

SUMMARY OF THE INVENTION

In accordance with the present invention, conventional controlparameters within a complex fractionation system can be augmented to asurprising degree through the generation of an overflash flow parameterwithin the withdrawal zone of the fractionation column. Such a controlparameter is generated by an overflash metering means comprising anotched weir means including upright wall means, positioned, say at theintersection of the withdrawal tray with downcomer means connecting theformer with a lower, adjacent fractionation tray. The function of thenotched weir means: to obstruct and then aid in measuring, continuously,the rate of flow of liquid overflash from the withdrawal tray to theadjacent fractionating tray via the weir means using differentialpressure measuring means provided with sensing means in the vicinity ofthe upright wall means. In turn, the differential pressure measuringmeans is connected in series to a recording means positioned external ofthe column.

In accordance with more detailed apparatus aspects of the presentinvention, the notched weir means preferably includes frame meansforming a central opening which can be placed in fluid alignment withthe downcomers attached to the withdrawal tray. The height of the framemeans above the withdrawal tray can vary as a function of circumferencealong the wall means, so as to form a series of uniformly spaced notchesat the upper edge of the wall means. In that way, overflash releasedthrough the notches and downcomers to the next adjacent fractionatingtray, can be measured as a function of the liquid head above thewithdrawal tray floor in the vicinity of the frame means, suchcharacteristic being accurately indicative of overflash flow even thougha portion of the liquid condensate is simultaneously withdrawn at thewithdrawal tray via side stream pump means in liquid contact with thesump of the withdrawal tray.

In accordance with detailed method aspects of the present invention, thedownwardly cascading overflash from the withdrawal tray to the adjacentfractionating tray provides a valuable processing control parameter whenused in association with a complex fractionating processing system, thatparameter being useful in controlling not only the flash distillationprocess interior of the column but in controlling exterior processes aswell. In this regard, the liquid head above the weir means within thewithdrawal zone has been found to be directly proportional to the flowof the overflash through the notched weir means and downcomers to theadjacent fractionating tray. Measurement of the liquid head is carriedout by differential pressure measuring means including sensor meanslocated at the intersection of the withdrawal tray and the downcomers.The latter generates a signal indicative of head, that signal beingrecorded by recorder means external of the column. Result: Overlimittrends related to the rate of flow of the downwardly cascadingoverflash, can be noted. And corrective action, as required, can then beinitiated to stabilize system operations.

FURTHER OBJECTS OF THE INVENTION

Further objects and features of the present invention will becomereadily apparent to those skilled in the art from the followingdescription of a preferred embodiment thereof, in which:

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a complex fractionation processingsystem utilizing an overflash measuring means of the present invention;

FIG. 2 is a top elevation of a complex distillation column of the systemof FIG. 1 illustrating constructional features of the overflashmeasuring means of the present invention;

FIG. 3 is a section taken along line 3--3 of FIG. 2;

FIG. 4 is a section taken along line 4--4 of FIG. 2;

FIG. 5 is a response curve of the overflash measuring means of FIGS.2--5; and

FIGS. 6 and 7 are an ensemble of histograms illustrating the usefulnessof the overflash measuring means of the present invention.

DETAILED DESCRIPTION OF A SPECIFIC EMBODIMENT OF THE INVENTION

Reference should now be had to the Figures, with particular reference tocomplex fractionation system 10 of FIG. 1. As shown, complexfractionation system 10 includes a complex distillation column 11. Acomplex controllor means 12 operatively connects to column 11, andincludes a separate regulation means 13 associated with control ofwithdrawal zone 14 of the column 11. Purpose of the controller 12 andregulation means 13: to optimize and stabilize proocess variables withinthe fractionating system 10 as a function of a various processparameters as explained in detail below:

In describing the operation of distillation column 11, consider the flowof feed, recycle, intermediate, and product streams at various inlet andoutlet lines of the column 11, e.g.,

1. Feed and recycle streams are seen in FIG. 1 to be associated with thelines 17-23 described as shown, while

ii. Intermediate and products streams are seen to exit from the column11 at overhead line 24, at medium gasoline draw line 25, at reboileroutlet 26, at bottoms line 27 as well as at side stream draw line 28.

As to flow in line 28, note the usage of a positive flow pump 29adjacent to the connection of the line 28 to sump 30 of the withdrawalzone 14 as well as the employment of a T-junction 31 at the dischargeside of the pump 29 which provide hot oil draw flows in two parallellines: junction line 31a carries the hot oil draw to vapor-liquidstripping system 32, and associated line 31b carries a parallel streamto hot oil circulation system 33. Stripper 322 of the stripping system32 provides a bottoms stream which becomes jet product via line 34 andpasses through a recorder analyzer 35.

Circulation system 33 performs two separate functions:

i. aids in the recovery of gas in the associated gas stream processconnected to line 31b in the direction of arrow 37; and

ii. controls the cooling by stream generation of the recycled hot oildraw stream within the secondary circulation networks 38a and 38b undercontrol of regulation means 13. As a result, the recycled oil reenteringthe column 11 via line 23 can, under normal process conditions, be keptwithin acceptable process temperature setpoint limits.

However, experience shows that even with the use of complex controlequipment, such as controller linkages 39 connected to controller means12, the fractionation system 10 of FIG. 1 can still become unbalanced.For example, if the withdrawal flow rate of hot oil via line 28 is toohigh (or too low), fractionation efficiency within the column 11 cansuffer before corrective operations can be initiated. Likewise, therecan be concomitant loss in efficiency within the processes linked to thecolumn 11, i.e., within liquidvapor stripping system 32 and withincirculation system 33.

In accordance with the present invention, process control parametersassociated with conventional complex fractionation system 10 of FIG. 1can be enhanced to a surprising degree by the generation andinterpretation of an overflash flow parameter related to excessoverflash flow occurring at the withdrawal zone 14 of the fractionationcolumn 11. Such a control parameter can be accurately generated by anoverflash measuring means 50 operationally attached to the column 11 asshown in FIG. 1. Briefly, the overflash measuring means 50 of thepresent invention provides a series of signals. These signals indicatethe rate of flow of overflash from the withdrawal tray 11a to theadjacent lower fractionating tray 11b via downcomers 3 and 4. Thesignals can then be recorded as a function of time to form histogramswhich can then be interpretated and corrective action taken, as by humanintervention, or the signals can be passed directly to the complexcontroller means 12, via complex linkage 39a, to effect direct changesin associated operating variables, say via selected sets of linkages 39connected from the controller 12 to various system control andregulation means, e.g., via linkages 39b-39e of regulation means 13.

FIGS. 2, 3 and 4 illustrate, in more detail, overflash measuring means50 of the present invention.

As shown in FIG. 3, fractionation in column 11 occurs at fractionationtrays 11b and 11c adjacent to withdrawal tray 11a. Vapor traffic fromthe fractionation tray 11b passes upward through the withdrawal tray 11avia chimneys to the fractionating tray 11c. Of course, the liquidtraffic is countercurrent to the vapor flow. It passes downwardly fromthe fractionation tray 11c to the withdrawal tray 11a via series ofdowncomers 7 fitted at the first-mentioned tray as shown in FIG. 3.Since the withdrawal tray 11a is solidly fastened as by welding to thesidewall 11d of the column 11, liquid that is not withdrawn via sump 30through line 28, must pass by the downcomers 3 and 4 to thefractionating tray 11b. Such flow only occurs if the depth of back-upliquid is sufficient to clear notched weir means 51 of overflash means50 aligned with the downcomers 3 and 4 at the intersection of the latterwith the withdrawal tray 11a. A differential pressure measuring meansgenerally indicated at 53 is also provided in the vicinity of notchedweir means 51. Its function: to translate differential pressures due tothe liquid head at the notched weir means 51 into pneumatic signals. Forthis purpose, it includes sensors 54 and 55 internally attached interiorof the column 11 at preselected positions, each sensor including nozzlemeans (not shown) via piping 56, 57, extending from the interior of thecolumn 11 to its exterior for attachment to differential pressuremeasuring cell means 58 connected in shunt between recording means 59and air supply 60.

The overflash measuring means 50 combines already-existingconstructional features of the withdrawal tray 11a with new operatingelements in order to provide an overflash flow control characteristicassociated with the withdrawal zone 14. In order to better appreciatethe present invention, a brief discussion of the constructional featuresof the withdrawal tray 11a is in order.

Note in FIG. 2 that the withdrawal tray 11a contains no sieve holes overits central region. Thus, in order to allow upward vapor traffictherethrough, a series of openings is provided through which the vaporchimneys 5 extend. A recessed channel member 6 of rectangularcrosssection is also provided over the central region of the withdrawaltray 11a, the channel member 6 forming the sump 30 of the withdrawalzone 14, as previously mentioned. At the remote termination of themember 6 is outlet line 28, the line 28 connecting to exteriorprocessing networks through pump means in the manner shown in FIG. 1.

Differential pressure measuring means 58 is only functionallyillustrated in FIG. 3 since it is readily available from severalcommercial sources. Briefly, in one embodiment, a pair of flexiblediaphragms (not shown) is used as a sensing element. These diaphragmsare welded to opposite sides of a rigid steel disc to form a forcebalance principle of operation of a differential cell 58a which allowsfor measurement of differential pressures in the range between 0-5 and0-25 inches of water differential at the withdrawal tray 11a. The outputpneumatic signal is generated by a pneumatic translation means generallyindicated at 58b. The generated signal proportional to differentialpressure at the tray 11b, say in a range of 3-15 psi, is thentransmitted to a recording means 59. At the recording means 59 there isgenerated a histogram of signal amplitude versus time calibrated asexplained below into flow information of the overflash interior of thecolumn.

In FIGS. 2 and 3 note the position of the sensors 54 and 55 of thedifferential pressure measuring means 53.

In FIG. 3, sensor 54 is located near the bottom of tray 11a whileterminal sensor 55 is located well above the tray 11a in the vapor zone,thus avoiding being submerged within the backup liquid in the vicinityof the downcomers 3 and 4. In FIG. 2, also consider that the terminalsensors 54 and 55 are vertically aligned in a common plane. Hence, theresulting indications of differential pressure are assured of accuratelymeasuring liquid head at the rectangular notched weir means 51 over arange of overflash flow rates convention in flash distillation processeswithout the introduction of variable turbulance or end effects whichcould, if uncorrected, distort such measurements.

FIGS. 2, 3 and 4 illustrate retangular notched weir means 51 in furtherdetail.

Since the functioning of the notched weir means 51 is the sum of theeffects of its left- and right-handed elements 51a and 51b, as viewed inFIG. 2, and, furthermore, since the operation of each element 51a and51b is identical one to the other, only the left-hand weir element 51awill be described in detail below.

As shown in FIG. 2, the notched weir element 51a is seen to include aframe means 70 defining an opening 71 having an axis of symmetry 72aligned with that of the adjacent downcomer 4. Frame means 70 also isseen to include a pair of parallel sidewalls 72 and much shortercross-connected end-walls 74. Each wall has a rectangular cross-sectionand is attached at its bottom edge 75 to the tray 11a, as by welding.The ratio of the lengths of the side and end walls are dictated, ofcourse, by the channel size of the downcomers 3 and 4 althoughvariations in construction can be implemented where required. In thisregard, note that in FIG. 2 the notched weir means 51 is not positionedadjacent to the sidewall 11d of the column 11, but is more centrallymounted near the axis of symmetry of the latter. Thus, turbulance andvariations due to end effects would be expected to be a problem;however, suprisingly, experience has shown that the flow informationprovided by the weir means 51, at least in the range of useful liquidflow traffic rates common to column 11, is highly accurate.

Since downcomers 3 and 4 have longer length dimensions than enddimensions, only the parallel sidewalls 72 of the notched weir means 51are provided--at the upper edges--with a series of uniformly spacedrectangular notches 77 (FIG. 2).

FIG. 4 illustrates notches 77 in more detail.

As shown, each notch 77 provides a rectangular notched opening 78.Opening 78 is defined by parallel sidewalls 79 and terminating end-wall80 and is spaced a uniform distance from its neighbor. The number andsizes of the notches 77 per sidewall 72 can vary, of course, dependingupon operating parameters within the column 11, and, moreover, depend toa great extent on the set point flow range of the overflash required forproper functioning of the column and its associated processes. In thisregard, where H_(o) is the incremental height of the sidewall 79 and Lis the wideth of end-wall 80, the following equation for volumetric flowhas been found to be of value in determining an acceptable flow range:

    Q = 0.386L . H.sub.o .sup.1.5 √ 2 g

Source: Chemical Engineers' Handbook, Perry, 4th Ed.

While a uniform series of rectangular notches 77 along each sidewall 72appears to provide better linear response characteristics in operation,it should be apparent that other types of notched weirs could also beused. In this regard, a vee notched weir means has been contemplated ashave other types of sharp-crested notched weir means such as found inElementary Fluid Mechanics, J. K. Vennard, 3rd Ed., at pages 312 et seq.Also of some value may be broadcrested notched weirs. However, since thelatter are more costly to manufacture and take more time to assemble,their probability of usage is somewhat lower than sharp-crested weirs.

FIG. 5 illustrates an actual response curve 85 generated by overflashmetering means 50 of the present invention.

Note that curve 85 is nearly linear over a selected overflashoperational flow range, i.e., curve 85 deviates only slightly fromtheoretical linear response curve 86 shown in phantom line over at anoverflash flow range of 20,000 BPD-90,000 BPD. Such responsecharacteristics seem to be due, at least in part, to the improve back-updepth of the liquid condensate within the withdrawal zone as provided bythe notches 77; such depth seems to provide a sufficient reservoir ofliquid to allow successful implementation of changes within variouscontrol parameters of the system to maintain high quality fractionationwithin the columns 11 as well as good withdrawal product quality withinthe associated processes. However, as operational requirements charge,it may be necessary to restructure the number of sizes of the notches 77to accommodate changed process conditions.

In constructing the curve 85, assume that a set of rectangular notches,each of which defines an opening equal to 2 × 6 inches, was used.Accordingly, for changed notch dimensions, any resulting response curvewould be shifted in proportional manner to the changed parameters overthose assocaited with curve 85 of FIG. 5.

Experience has shown that in a complex fractionation system of the typeillustrated in FIG. 1, the overflash metering means 50 of the presentinvention, having the following dimensions and specifications, canprovide surprisingly useful sets of control parameters.

Overflash metering means 50

Rectangular notched weir means 51a

Frame means 70

All over dimensions, 15 inches × 1 inch

Height of side and end walls 72, 74: 10 inches

Rectangular notch 77

Dimensions: 2 inches × 6 inches

Number: 10

Differential pressure means 53

Pressure differential ranges 0-5 and 0-25 inches of water; staticpressure max. 500 psi,

Output signal: 3-15 psi.

Manufacturer: Foxboro Corporation

Model 15

Air Supply 60

Pressure Rating: 20 psi

Flow Recorder 59

Type: Pneumatic

EXAMPLE I

FIG. 6 is an ensemble of histograms 90, 91 and 92 generated by varioussytems recorded controllers within a complex fractionation system of thetype illustrated in FIG. 1.

The geneses of the histograms 90, 91 and 92 were as follows: Histogram90 was generated by the overflash measuring means 50 of FIGS. 1-4;histogram 91 was produced by recorders 40 and 41 of FIG. 1; andhistogram 92 was provided by analyzer recorder 35 of FIG. 1.

With specific reference to histogram 90, note a linear steady outputcurve 93 has been generated at about 80% full overflash flow: i.e., atabout 68,000 BPD of overflash for a 7-hour time period. Likewise, inhistograms 91 and 92 for the same time period, there are indicationsthat the fractionation system has generated high quality distillates andside-stream products.

For example, reflux overhead flow rate 94 in histogram 91 (which iscontrolled by the mid-gasoline draw temperature curve 95) is seen to beat a high value, i.e., it shows an overhead reflux rate of about 27,000BPD's at a medium gasoline average draw temperature of about 320° F.Similarly, the 10% and 90% distillation product curves 96 and 97 ofhistogram 92 are also well within operating setpoint limits. That is,the curves 96 and 97 are between 300°-400° F. and 400°-500° F., thesetpoint temperature ranges; say at about 385° F. (curve 96) and 428° F.(curve 97), respectively.

EXAMPLE II

FIG. 7 illustrates a second set of histograms 100, 101 and 102 for a24-hour period in which process upsets occur as predicted by theoverflash metering means of the present invention.

In more detail in FIG. 7, the histogram 100 for the overflash measuringmeans of the present invention indicates a curve 103 that shows, for amajority of the represented time period, a rather low rate of flow ofoverflash even though within acceptable setpoint limits, i.e., about22,000 BPD. However, at region 104a of the curve 103, the flow rate asindicated by overflash meter means falls below acceptable limits, i.e.,from about 17,000 BPD to zero.

Indications of system inefficiency are also shown in histograms 101 and102. For example, in histogram 102, reflux overhead rate 105 is unsteadyfor similar time periods and seems to reach a low rate at approximately10,500 BPD in the region 104c aligned in time with region 104a ofhistogram 90, although the medium gasoline draw temperature curve 106seems to be within specification limits, i.e., at an average temperatureof 310° F. where the setpoint range is about 250°-350° F. Jet productcurve 107 (10%) of histogram 101 also is seen to be on the low side of,but within, the setpoint temperature range of 300°-400° F., particularlyin the region 104b aligned in time with the regions 104 and 104c of thehistograms 100 and 103 respectively.

When an overflash flow rate as indicated by the histogram 100 wasrestored within acceptable setpoint limits, say be reducing withdrawalof the liquid condensate from sump 30 of withdrawal tray 11a via sumpmeans 29 in line 28 (FIG. 1), the system returned to efficientoperations.

While specified preferred embodiments of the present invention have beenhereinbefore described, it should be understood that the invention isnot limited thereto as many variations will be readily apparent to thoseskilled in the art and thus the invention is to be given the broadestpossible interpretation within the terms of the following claims.

We claim:
 1. An article of manufacture to increase condensate flowinterior of a fractionation column at a withdrawal tray adjacent to afractionating tray connected thereto via a downcomer means having afour-sided, raised lip, rectangularly shaped in cross section, extendingabove said withdrawal tray defining a vertical opening therethroughcomprising, in combination, a four-sided upright barrier wallrectangularly shaped in cross section to match that of said raised lipof said downcomer means, said upright barrier wall further comprisingmeans cooperating to connect and means cooperating to extend it upwardlyfrom said raised lip of said downcomer means and including a series ofnotches along its top extremity away from said downcomer means, suchthat condensate flow between said withdrawal and said fractionatingtrays of said column is directable from locations exterior of saidbarrier wall through said series of notches along at least two sidesthereof to a position interior thereof so that said flow can be measuredas a function of back-up depth of said condensate exterior of saidbarrier wall away from said downcomer means over a range of flow ratesassociated with efficient column operations.
 2. Article of claim 1 inwhich said notches of one side of said barrier wall are rectangular incross section along a vertical plane therethrough, whereby said flow ofcondensate is approximately a linear function of said back-up depth ofsaid condensate exterior of said barrier wall, over a range ofmeasurable flow rates associated with efficient column operations. 3.Article of claim 2 in which said series of rentangular notches arelocated along said top extremities of said two sides of said barrierwall, said two sides forming opposite long sides of said rectangularlyshaped in cross section barrier wall whereby a maximum number of notchescan be formed therein to direct large amounts of condensate flowtherethrough allowing efficient column operations.